1. Field of the Invention
The successful, profitable growing of tobacco (Nicotiana tabacum L.) requires great skill at judging the proper stage for various decisions and taking such action in a timely manner. Flue-cured tobacco is the most important segment of the total crop in the United States. Tobacco must reach a particular level of maturity before it is harvested. A tobacco leaf is considered mature when among other characteristics, the lamina (i. e. leaf blade area between veins) begins to develop a slight yellow color. The tobacco leaves are removed, either mechanically or by hand, from the stalk in sequential harvests and each pass consists of three to six leaves. There will usually be a total of 16 to 22 harvestable leaves per stalk. Flue-cured leaf is cured by being subjected to artificial heating and ventilation in a barn especially designed for that purpose. The first step of the curing process is to further develop the yellow color of the leaf, which requires about three days, or half of the normal curing period. If the tobacco has not reached the correct level of maturity before being harvested and put in the barn, the tobacco will not cure properly and will lose market value. Barn capacity is one of the larger costs to the grower; therefore most growers do not have an excess of barn capacity. Heating fuel used to cure the tobacco is also a major cost to the grower. Therefore to have a profitable operation, the grower must harvest the tobacco at maximum ripeness to conserve fuel, and the grower must allow his tobacco to ripen in accordance with a schedule that allows a barn to always be available when tobacco is ripe enough for harvest. However for efficient utilization, the barns should be used for as many curings per season as possible. Tobacco ripens best when water and nutrients are much less available in the soil after all leaves are fully expanded. Consequently, ripeness may be particularly delayed by excess nitrogen and moisture, and to a lesser extent by other nutrients in the soil, as the tobacco crop approaches the projected harvest date.
As our agricultural production has become more intensified, these uncertainties relating to maturity have been addressed by various means; of particular interest are preharvest curing aids which have been developed, which are described in E. B. Whitty, et al., 1992, Enforcement Procedures Result in Label Compliance in Use of Chemicals for Yellowing Tobacco, Proceedings of Soil and Crop Science Society of Florida, Vol. 52, pp. 14-17, which is hereby incorporated by reference in its entirety. These developments have allowed the tobacco grower to exert greater control over time of harvest and time of placing the crop in the barn for curing. This has allowed more efficient use of fuel and barn capacity. The compound 2-chlorethylphosphonic acid is the only commercial product labeled by the EPA and in use for this purpose. Possible disadvantages of 2-chlorethylphosphonic acid are that effective use requires about 500 grams per acre or more and many tobaccos are reduced in market quality by the usual change to a more orange color that it produces. Another possible disadvantage of 2-chlorethylphosphonic acid is that it is perhaps an alkylating agent. The 1992 Whitty reference also describes the illegal (in the sense that they were used not in accordance with their EPA labels) use of various synthetic auxin herbicides as preharvest curing aids at rates of 250 to 500 grams per acre. From the standpoint of label compliance it is unfortunate that these synthetic auxin herbicides produce a more natural appearing ripening of the tobacco, both in the field and after curing in the barn than does 2-chlorethylphosphonic acid. An important disadvantage of the synthetic auxin herbicides as preharvest curing aids is that they produce to varying degrees epinasty of the treated leaves. Epinasty is more vigorous growth of the upper leaf surface relative to the lower leaf surface, with resultant puckering and malformation of the leaf; it is characteristic and diagnostic of both synthetic and natural auxins. This epinasty is always quite apparent to the grower. Tobacco growers are normally very traditional and very conservative. Tobacco growers would find this epinasty associated with the use of synthetic auxins quite unacceptable, even though there is no intrinsic problem with curing such treated leaf; it turns out very high quality tobacco from the barn after curing. Massive education efforts would probably be necessary to convince growers to accept these synthetic auxins as preharvest curing aids for tobacco.
The methods for preharvest curing aids to be disclosed in this application are advantageous over the prior art 2-chloroethylphosphonic acid methods in that the treated leaf is much more natural in appearance and could be expected to develop into cured leaf earning a much higher price in the market. The compounds of the new methods also require a much lower application rate than prior art 2-chloroethylphosphonic acid and unlike 2-chloroethylphosphonic acid, are clearly not alkylating agents. The methods for preharvest curing aids to be disclosed in this application are advantageous over the prior art illegal (i. e., used out of label) synthetic auxin herbicide methods in that the treated leaf is much more natural in appearance and could be expected to develop into cured leaf earning an unusually high price in the market. This is because the new methods do not produce leaf epinasty and would be immediately accepted by the typical conservative, traditional tobacco grower. The methods for preharvest curing to be disclosed in this application are also advantageous over the prior art methods in that they require a much lower application rate than prior art illegal (i. e., used out of label) synthetic auxin herbicide methods.
2. Related Art
The related art is well described in E. B. Whitty, et al., 1992, Enforcement Procedures Result in Label Compliance in Use of Chemicals for Yellowing Tobacco, Proceedings of Soil and Crop Science Society of Florida, Vol. 52. pp. 14-17, which has been incorporated by reference in its entirety. There is no prior art reference to the compounds useful in this invention as preharvest curing aids for tobacco.